1. Field of the Invention
The invention relates to a proximity sensor adopted in applications of determining the presence or absence of a metal body and measuring a distance of the metal body therefrom using a change in oscillation amplitude of an oscillation circuit including a coil.
2. Description of the Related Art
There have been available proximity sensors of this kind in each of which a range (detection range) over which a metal body as a detection object (hereinafter referred to as “body”) can be detected is variably set by an adjustment operation of a user. As a typical example, a variable resistor is incorporated in an oscillation circuit and a rotational operation is conducted on the variable resistor to thereby change a current flowing in a feedback circuit of the oscillation circuit.
In Japanese Patent Publication No. 3440566, a proximity sensor with the construction is presented as a conventional example and, besides, a proposal is made on a sensor of a type with improvement in the construction. The sensor of an improved type includes: a sensitivity adjustment circuit having plural series circuits constituted of a resistor and a switch, wherein change-over between combinations of resistors used in sensitivity adjustment is conducted by control with a microcomputer to thereby control a feedback current quantity supplied to a resonance circuit (see FIG. 1 of Japanese Patent Publication No. 3440566). Besides, in the publication, another proximity sensor is also disclosed in which change-over is conducted between resistors in a current mirror circuit determining a feedback current to thereby control a feedback current quantity (see FIG. 15 of Japanese Patent Publication No. 3440566).
Since a relationship between a rotational quantity of a variable resistor and a detection distance is nonlinear in a case where a magnitude of a feedback current is adjusted by a rotational quantity of a variable resistor, a state arises where difficulty is encountered grasping an operational amount necessary to acquire a desired detection distance. Hence, an operator is required repetition of an adjustment operation and confirmation of action of a sensor due to the adjustment, having led to a problem to consume labor and time in the adjustment.
In a proximity sensor of a type adjustment operation of which a user conducts, properness of the adjustment is judged by the intuition or experience of the operator and there is not shown any of indexes each of which is used for determining whether or not an adjusted oscillation amplitude has a sensitivity suitable for detection of a body.
In a case where proximity sensors with the same performance are arranged and the same detection distance can be commonly set to the sensors, sensitivities of the sensors are desirably made uniform. Since a conventional sensor adjustment of which is conducted with a variable resistor is provided with no contrivance enabling an operational quantity to be correctly grasped and there is possibility of causing a variance in the adjustment even by a slight hand movement, difficulty is encountered conducting uniform adjustment. Even if one expert conducts the adjustment in the sensors, it is difficult to avoid generation of a variance in sensitivity over the sensors due to the same reason.
On the other hand, according to a proximity sensor disclosed in Japanese Patent Publication No. 3440566, since a combination of resistors is determined by a microcomputer, many of resistors provided enable proper sensitive adjustment to be implemented depending on a detection distance desired to be set. If many of resistors are provided, the number of parts increases, resulting in a problem of a up-sizing of the equipment and increase in cost. Even this proximity sensor has no means notifying a user of how a sensitivity is set.